Several operators are understood to have complained to the EU in recent months, and a spokesman for the competition commissioner, Joaquín Almunia, said on Friday that the regulator would intervene if there were indications of anti-competitive behaviour.

"The commission has been made aware of Apple's distribution practices for iPhones and iPads," he said. "There have been no formal complaints. The commission is currently looking at this situation and, more generally, is actively monitoring market developments. We will intervene if there are indications of anti-competitive behaviour to the detriment of consumers."

Apple imposes stricter terms on networks that wish to sell its phones on to their customers than other handset makers. There are minimum orders, and if the network does not sell a pre-agreed number of iPhones within a set time, it is liable for unsold stock. A source at one telecoms company said: "They have all kinds of different rights including, if you aren't selling enough, that they step in to take over your marketing and you pay them for that. It's great when the phones fly off the shelf, but if they come up with a dud product you could be left with a lot of cost."

An Apple spokesman said: "Our contracts fully comply with local laws wherever we do business, including the EU."